CHAPTER FOUR
Ringplain and Steeplands
T~~d~:~a;:~ki-~angan ui region mainly consists of two strikingly different
o
o
the intensively farmed volca nic landforms of western Taranaki, surrounding
the classical young cone of Mt Taranaki (or Mt Egmont) (Plate 4.1); and
the sparsely populated, deeply dissected hill country of eastern Taranaki and
the Wanganui River catchment.
In addition, two sets of uplifted marine terraces form a coastal fringe - from
Urenui to New Plymouth, and from Hawera to Wanganui (Fig. 4.1). The contrasting
geology of the different landforms is reflected in their drainage patterns. Whereas
hundreds of small, entrenched streams simply radiate out from the Taranaki
volcanoes, the east Tara naki-Wanganui hill country has a complex network of
streams, often called a 'dendritic' drainage pattern since it resembles the branches
of a tree.
The region is exposed to the prevailing mOisture-bearing westerly winds and
the slopes of Mt Taranaki are the wettest part of the North Island (up to 8000 mm
of rainfall annually); the rest of the region, except the drier southern coastal terraces,
receives 1400 -2000 mm. Despite this high rainfall, the streams draining western
Taranaki have very limited floodplains and erosion of the volcanic soils is relatively
mmor.
Moderate to severe soil erosion does, however, occur in the east TaranakiWanganui hill country because of steep slopes, forest removal, and the inherent
suSceptibility of the mudstone, siltstone and sandstone rocks to slip erosion. Much
of the Wanganui River catchment (over 700 000 hectares) is still forested, although
there are many areas of reverted farmland testifying to the difficulty of farming
the steep, unstable soils around its middle reaches. The river was the main access
route to the interior in historic times. Today it is prized as a scenic waterway,
rich in Polynesian and European history and the land around the river has now
been designated a national park.
The Volcanic Landscape of Taranaki
Although Mt Taranaki dominates the Taranaki landscape, it is only the most recent
of a remarkable series of andesitic volcanoes that have developed in western
Taranaki. The oldest remnants are the monoliths of Paritutu and the Sugar Loaf
Islands on the outskirts of New Plymouth. The forested Kaitake Range is the deeply
eroded stump of an ancient volcano nearly 600 000 years old. Around 250 000
years ago activity moved south-east, to where Pouakai volcano now stands on the
dissected remnant of a cone once as large as Mt Taranaki is today. Mt Taranaki
itself probably began to be formed around 70000 years ago and continued to
erupt intermittently until its last recorded eruption about A.D. 1750.
As each volcanic cone was built up by successive eruptions, natural e rosion
stripped away much of the volcanic debris to redistribute it as an apron (termed
a ringplain) around its base. Tephra deposits mantled the land as described in
Chapter 2 but avalanches of cold volcanic debris were also common. Over the
last 50000 years the cone of Mt Taranaki has intermittently collapsed causing
huge very mobile debris flows Qahars) to sweep down the mountain as far as the
sea on the western slopes. While the popular image oflush Taranaki dairy pastures
71
Plate 4.1 (opposite)
The snow-covered cone of Mt Taranaki
(or Mt Egmont) dominates this typical
Taranak i pastoral landscape near
Stratford. At this point on the eastern
rim of the Mt Taranaki ringplain, the
soils are mapped as Stratford soils
(Plate 4.6) which have developed in the
Stratford Ash shower.
72
The Living MamIe
~km
Fig. 4.1
Drainage pattern of the Taranaki~
Wanganui district showing rivers and
streams radial to the volcanoes of
Mount Taranaki and Mount Ruape hu
This is in marked contrast to the
dendritic pattern of rivers in the
Wanganui hill country between these
two volcanic centres.
South Taranaki-Wanganui
upliftedmarinelerraces
on deep free~draining tephra soils applies east of Mt Taranaki, many of the soils
on lahars to the west of Mt Taranaki are very sto ny and suffer from impeded
drainage.
In contrast to the Violently exploSive eruptions of the Taupo Volcanic Zone
(Chapter 2), the andesitic tephras in Taranaki accumulated more gradually from
many small eru ptions. Up to 30 m of tephra has accumulated over 100000 years,
an average of only 0.3 mm annually. In addition, there are many well-developed
buried soils (paleosols) indicating considerable periods without the addition of fresh
tephra.
The volcanic soils on the ringplains of the old Pouakai volcano and the presentday Mt Taranaki have developed within the past 23 000 years, through eruption
(and subsequent erosion) of tephras from Mt Taranaki. Whereas the Pouakai
ringplain is covered only with airfall tephras, the Mt Taranaki ringplain is a complex
mixture: here the earlier tephras have been buried by subsequent lahars which
have then been covered in turn by further local tephra eruptions. As with any
volcano, the thickness o f airfall tephras depends upo n the distance from the source
- as well as the wind direction. In Taranaki the westerly and southerly winds
have resulted in the thickest deposits of airfall tephras in the northern and eastern
sectors.
In summary, the Taranaki volcanic soil pattern depends upon the age (or
presence) of lahar deposits, and the thickness of any airfall tephras on the lahars.
Floodplains are non-existent or very narrow. Alluvial processes are important only
in the lower reaches of the Stony River and Mangahume Stream (Hangatahua soils).
Since the major airfall tephra deposits vary in age from 5000 - 25 000 years they
have weathered sufficiently to exhibit the pedological character of volcanic Ioams
(see Chapter 2).
RingpJain and Steeplan&,
73
Plate 4.2
Soil Pattern across the Mt Taranaki Ringplain
The relationship of some of the soils to the volcanic landforms and rainfall is shown
in Fig. 4.2. This particular cross-section of the symmetrical ringplain of Mt Taranaki,
from Cape Egmont to Stratford, illustrates the volcanic loams which have developed
in the thick (Le. over 2 m in depth) andesitic tephra deposits. If a cross-section
from Opunake to Inglewood had been chosen, it would have shown the soils on
the south-west sector of the ringplain. These Opua, Te Kiri and Pihama soils have
only a very thin cover of volcanic ash because they developed on extensive, stony
lahars which swept down to the lowlands after the main soil-forming tephra
eruptions.
The ringplain on the western side of Mt Taranaki is dimpled with large numbers
of laharic mounds (plate 4.2). These deposits are stony and bouldery, and are covered
by a layer of volcanic ash which varies in depth from 60 cm near the coast to
over 2 m on the lower slopes of Mt Taranaki. The deposits are generally cemented,
impeding soil drainage in low-lying areas as well as forming a barrier to roots.
Nearer the coast, Awatuna soils in the poorly drained flat surfaces between the
Between Warea and Rahotu on the
western side of Mt Taranaki, about
20 000 ha of the ring plain are dimpled
with thousands of laharic mounds. This
very distinctive topography is the result
of water-saturated rock and tephra
debris flowing down from the higher
slopes of the volcano. Most of the
mounds in this view looking eastwards
towards the Pouakai Range (left) and
Mt Taranaki were deposited
23 000 - 22 000 years ago. The soil
pattern is complex, with most of the
foreground landscape mapped as an
association of Warea hill soils
(Plate 4.3) on the lahar mounds, and
the gleyed Awatuna soils in the
intervening flats and hollows. This soil
association is of moderate value for
food production and is mainly used for
sheep and cattle grazing and dairying.
74
The [ivins MamIe
Laharsurtace
Occasional lahars
Silty sandstone
Volcanic rocks (lava,
laharic debris)
Airtalltephra
Annualrainlall
(mm)
8000
2000
1-------
Altitude
1m)
i
w
Fig. 4.2
Soi l/ landform and rainfall cross -section,
from Cape Egmont to east Taranaki hill
country.
10km
'-----~~---',
"
Toko
East Taranaki
hill country
'{l
mounds are gleyed w he reas th e Warea hill soils (Plate 4.3) on the slopes of th e
mounds are free draining. Furthe r inland, the incidence of lahar mounds is greater
and Warea hill soils predom inate.
The land-use ve rsatility of these areas is limited by the intricate pattern of
laharic mounds which restricts the use of agricultural machinery and makes the
establishment of drainage networks difficult (plate 4.2). Above about 160 m altitude,
Kahui hill soils occur on the lahar mounds (plate 4.4); these soils have similar
properties to the Warea hill soils (Le. well drained, loamy, friable) but their subsoils
are duller in colour and they are more leached because of the wetter, cooler
environment.
Above an altitude of around 400 m the laharic surface is much smoother and
the well-drained, strongly leached Patua soUs (plate 4.5) occur on the tephra-covered
slopes. At this higher altitude the climate is cool and wet and, because the tephra
is coarser (closer to its source), the soils are more susceptible to erosio n if vegetation
is disturbed. O utside Egmont Natio nal Park, Patua soils are used for dairy fa rming
but fertilise r maintenance levels are high because of leaching. At higher altitudes
Patua soils give way to coarse-textured, very strongly leached soils on relatively
unstable volcanic debris (fahurangi and subalpine/alpine steepland soils).
Some of the poorly drained soils on the western sector of the Mt Taranaki
ringplain have hard iron oxide layers (known locally as 'ironstone) in their subsoils
probably resu lting from lateral seepage of iron dissolved from the andesitic rocks
highe r up on the volcano . At low altitudes these seepages are forced to the surface
and precipitate out as iron oxides where they come in contact with air. Higher
on Mt Taranaki, iron-rich groundwaters have deposited an orange ochre (kokowai)
in the bed of the Kokowai Stream. This deposit was prized by the local Maori
people as a source of ochre for ceremonial and artistic uses.
The lahar mound topography is absent from the eastern side of Mt Taranaki;
rainfall is high (Fig. 4.2) and tephra deposits dominate the soil profiles. Stratfo rd
RinBpJain and Steeplands
75
soils (Plate 4.6) are the most widespread. They have developed on a group of rather
coarse tephras collectively termed the 'Stratford Ash', which was probably erupted
about 7000 years ago. They are free draining, an advantage in th is high-rainfall
area and, despite nutrient deficiencies, they are ideal soils for dairy farming 0
(Plate 4.1). East of Stratford, where the Stratford Ash is further from its source,
~~;;:a:~:ds:~~~}:;~ ~~~r tt~~~~~t.(fi:l~;:nud~d~:¥~~n!;~::~i::~: ~~ ~~ff~:.~
r
l
become soil forming. These soils are finer textured with a strongly structured A
hOrizon, and a friable B horizon overlying a dark yellowish-brown tephra wIth .Im
a greasy fe.el and clay loam texture. In their properties they are similar to the
Egmont sOlis, which are extensive to the south around Hawera, and to the New
Plymourh soils of western and norrhem Taranaki; they are of high value for cropping
and pastoral uses.
1m
East Taranaki-Wanganui Hill Country
East of Toko, the ringplain of Mt Taranaki gives way to the strongly dissected
hill country of east Taranaki. The underlying strata are not volcanic, but consist
of Tertiary sedimentary rocks (mudstones , siltstones and sandstones) which in places
still carry a covering of andesitic tephra. Further east lies the vast maze of the
Matemateaonga Range and the Wanganui River catchment. In addition to the
distinctive dendritic drainage pattern of this country, there is a uniform height
to the hill crests (suggesting a former marine terrace surface which was later
dissected) and a symmetry of angle in the development of ridges, spurs and valleys.
Soil properties are closely related to differences in rock hardness and
composition; most are steep/and soils (see p. 81), i.e. rather shallow soils which
have developed on steep. relatively unstable slopes. Often, erosion has prevented
the development of the typical profile features of a mature soil. The landscape
of the Whangamomona steepland soils (Plate 4.7) and the profile of a Tahora
steepland soil (Plate 4.8) are illustrated. They are only two of nearly 40 different
hill and steepland soils which have been recognised throughout 500 000 ha of
this rugged east Taranaki-Wanganui sedimentary hill country.
The east Taranaki and mid-Wanganui hill country has had a long history of
agricultural depressions and personal hardship for the scattered farming
communities. With the extension of the railway to Ohura, many small settlements
developed and Crown Land leasehold blocks were settled in the optimistic days
following World War I. Most of these farms were abandoned in the 1920s and
1930s, the land reverting to scrub and second-growth forest.
The pattern is not very different today. The combination of steep topography,
a cool, wet climate, and relatively shallow, erosion-prone soils makes most of the
area suitable for store farming only. Production levels (including lambing
percentages) are low by national standards since pastures are generally of poor
quality. The farmers' constant battle to maintain adequate grazing is a hard one
in the face of the natural tendency for reversion, and as a result stock (predominantly
cattle) are underfed. The isolation and difficult access into this large area of hill
country are major factors limiting attempts to sustain productive use of the land
(Plate 14.7). Over 7000 ha of reverting land have been successfully converted to
exotic forest plantations in Te Wera State Forest 30 km inland from Stratford,
but the same factors of steep topography, erosion and isolation limit the prospects
for any widespread commercial forestry options.
This wide belt of sedimentary hill country extends well to the east of the heavily
forested Wanganui catchment, into the Whangaehu, Turakina and Rangitikei
catchments. Here the environment is slightly drier, and its soils are discussed in
Chapter 6.
Soils of the Taranaki Coastal Terraces
The soils of the marine terraces (and the old Pouakai ringplain) to the north and
south of Mt Taranaki are the most versatile and productive in the region. These
flat to gently rolling surfaces lie beyond the Mt Taranaki ringplain with its
entrenched streams and bouldery lahars. The tephra depOSits on the old terrace
surfaces are quite deep and, because they are further froIT! the volcanic centre,
are fine textured. In the south, the Egmont soils generally consist of at least 1.5 m
Plate 4.3
Warea hill soils are moderately leached,
well -drained, friable, loamy textured
soi ls which have developed on lahars
of volcanic debris subsequently
covered with andesitic tephra. The
poorly sorted nature of the laharic
debris ranging in size from sands,
throug h stones to very large boulders is
evident below the mantle of 90 cm of
airfall tephra deposited over the last
23 000 years
76
The LivinB Mantle
Plate 4.4
View eastward towards the Pouakai
Ranges (left) and Mt Taranaki (right).
from near Newall Road, at 300 m
elevation. Kahui hill soils occur on the
laharic mounds and Kahui soils on the
undulating land between. Newall soils
and the gleyed Awatuna soils occur on
the flat land in the middle ground and
in low-lying land between the laharic
mounds. At around 400 m elevatio n,
the farmland passes into indigenous
forest at the Egmont National Park
boundary. Patua soils (Plate 4.5) occur
on the Pouakai Ranges at this altitude,
and Maero soils occur on recent
colluvium on the slopes of Mt Taranaki
Plate 4.5 (right)
The Patua soils are very strongly
leached volcanic loams which have
developed in deep andesitic tephras at
altitudes of 400 ~ 900 m (annua l rainfall 0
2000 - 4000 mm) on the western and
northern sectors of the ring plains of Mt
Taranaki and Pouakai volcano. They
have the t.Y.~ical friability, I~amy texture
and free drainage of volcaniC loams;
bu lk densities are very low, phosphate 5m
retention is very high. and most of the
clay fraction consists of allophane
(With. minor .amoun~s .of vol~anic g.lass).
A feature of the soli IS the IIluviallfon
oxide deposited as dark reddish -brown
coatings around 60 - 90 cm depth.
1m
r
·l
Plate 4.6 (far right)
Stratford soils are moderately leached
volcanic loams from andesitic tephras,
which cover a large area of the lower
eastern slopes of the Mt Taranaki
ringplain from Eltham to Inglewood.
They are formed in younger tephras
(Stratford Ash) than the Egmont and
New Plymouth soils (Plates 4.9 and
4.10) and the presence of pumiceous
lap illi gives them a coarser texture
RinspJain and SteepJands
77
Plate 4.7
Th e la ndscape of the Whangamomona
st eepland soi l mapping unit east of
Doug las in t he east Taranaki hill
cou ntry . Th e soil pa rent materia l is
weat hered silty sandstone; any t ephra
depos ited on t hese slopes has long
sin ce been eroded away. T he
unifo rmity of height to the hi ll crests
probably indicates the former existence
ofa flat marine terrace surface, which
was subsequently dissected by
deg rading rivers.
78
The Living Mantle
The Tahora steepland soils are found
on large areas of sandy mudstone
north of Whangamomona, and
eastwards in the Aotuhia district
towards the Wanganui River. They are
olive brown in colour and have heavier
textures (silt loam or clay loam) than
the volcanic ash soils to the west.
Strongly developed nut structures are
common, as is the presence of
mudstone fragments, especially in the
subsoil
Plate 4.9
(above right)
The Egmont soils have 25 cm of black
(or dark brown) A horizon over a
friable, loamy, brown upper B horizon
(45 cm), over a paler yellowish-brown
lower B horizon of silt loam texture.
The black topsoil of this Egmont black
soil probably originated from fire
induced flax lPteridium fern vegetation
during Polynesian settlement along the
approximately 4-km wide coastal
fringe.
of andesitic tephra over tephric loess or old dune sands. The Aokautere Ash (the
20000-year-old rhyolitic tephra from the Taupo Volcanic Centre, Fig. 2.2), is
commonly found at a depth of about 2 m in these subsurface materials.
The Egmont soils are classic volcanic loams (described in Chapter 2), with a
high a1lophane content, high porosity and a friable consistence. Two types of Egmont
soils are distinguished: the Egmont black soils (Plate 4.9) near the coast, and the
Egmont brown soils which developed further inland under a podocarplhardwood
indigenous vegetation . Although dairy farming is still the predominant use of the
Egmont soils, the drier climate (1000-1200 mm annual rainfall) along the south
Taranaki coast has encouraged some farmers to convert to cash cropping. Nearly
2000 ha of maize, wheat and barley grain is now harvested but this is only about
20 percent of the regional requirement for feed grains. The Egmont soils are also
suitable for horticulture (berry fruit, nurseries, market gardening) but the
establishment of shelter against the prevailing westerly winds is essential, particularly
on the Egmont black soils where salt damage can be severe.
In the north the New Plymouth soils are similar moderately leached volcanic
loams. However, they have a much thicker (up to 15m) depth of tephras, including
many older tephras derived from the Pouakai volcano (Plate 4.10). The New
Plymouth soils are also slightly warmer than the Egmont soils and are considered
to lie just inside the thermic soil temperature zone (Table 14.1). The Egmont soils
lie within the warm soil temperature zone (see Fig. 14 .3).
Ringplain and Steep/and5
79
Plate 4.10
The New Plymouth soils are moderately
leached, deep, free -draining soils which
have developed in the airfall andesitic
tephras mantling the marine terraces
and parts of the Pouakai ringpla in in
north Taranaki. The soil shown is the
New Plymouth brown soil, indicating
the influence of a podocarp/hardwood
forest vegetation cover in the
development of the topsoi l colour (et
Egmont black soil, Plate 4.9 )
Most of the New Plymouth soils are
used for dairy farming but horticulture
is beginning to change the landscape
along the marine terraces from New
Plymouth to Waitara (Plate 14.13) .
These soi ls were used to establish
1000 ha of the indigenous shrub
Poroporo (Solanum av;culare) in a
commercial attempt to extract steroids
for the pharmaceutical industry.
Plate 4.11
Traditionally the New Plymouth soils have been used for dairy farming - like
most soils of Taranaki. The usual dairy farm is around 60 ha and carries 100 - 140
cows. Pasture productio n is around 12 000 - 14 000 kg dry matterlha/year which
can realise 350 - 450 kg milkfatlha from the predominantly Jersey and Friesian herds.
Although they are ideal pasture-producing soils (once phosphorus and potassium
limitations have been overcome by the application of potassic superphosphate
fertilisers), they are probably the largest group of soils with high potential productivity
for various forms of cropping and horticulture. Although a thriving local nurseryplant industry is of long standing (particularly around Waitara, Plate 14.13) the
Taranaki agricu ltural community has only recently begun to recognise the versatility
of the New Plymouth soils.
The Waitotara River is one of many
rivers and streams which have
dissected the south Taranaki marine
terraces. Egmontand Westmeresoils
(andesitic airfall tephras and loessial
parentmaterialsloccuronthemarine
ter race (background) and free-draining
Manawatu soils on the lower alluvial
terrace close to the river. Shelterbelts
are being established on the alluvial
terrace for horticulture (kiwifruit
vineyards) and there is an exotic forest
plantation on the Kumeroa hill soils on
the spur in the mid dle distance. The
Kumeroa hill soils have developed from
the sedimentary rocks (silty
sandstones) underlying this landscape,
and the alluvial Manawatu soils from
the erosion products of these rocks.
80
The [ivins Mantle
Plate 4 . 13
The Westmere soi ls cover an area of
around 9000 ha on the marine terraces
in the Kai Iwi and Wanganui distri cts.
They have developed in a drier
environment (annual rain fall
900-1300 mm) than that of the
typical volcanic loam s of Tara naki
They are chiefly used for pastoral
purposes but their properties of good
structure, consistence and dra inage
make them of va lue for cash cropping
and horticulture (berry fruit, kiwifruit
and market garden ing).
The marine terraces between Hawera and Wanganui were dissected before
the depositio n of most of the tephras. A typical landscape is that shown in Plate 4.1 I
around the Waitotara River - dissected marine terraces resulting in interfluves
with steep valley sides and flights of alluvial terraces in the valley itself. Egmont
soils predominate on the marine terraces as far south as the Waitotara River, but
from here to the Wanganui River the Mt Taranaki tephra showers of the last 20 000
years rapidly thi n out. Instead, older andesitic tep hras mixed with loess (of both
andesitic and quartzo-feldspathic origin) are the parent materials of the Westmere
soils.
The Westmere soils (Plate 4.12) are weakly leached, deep, friable, well drained
and well structured. Their natural nutrient status is moderately high and they have
high moisture· holding capacity. They are versatile soils particularly suitable for
cropping and market gardening (Pl ate 4.13). Although they are similar in many
ways to the Egmont and New Plymouth soils, they are less friable, have finer textured
and stidder subsoils, and contain the day mineral halloysite in addition to alIophane.
In these properties the Westmere soils are intermediate between the volcan ic loarns
and volcanic loamy d ays of the so uth Auckland district (see Chapter 3).
Wanganui marks the south-wes tern extent of soils dominated by tephras from
the volcanic plateau, just as the Manukau Harbour is their northernmost limit.
The jo urney down from the pumice plateaux to the loamlands around their
periphery has traversed the four soil groups - raw volcanic soils, pumice soils,
volcanic loarns, and volcanic loamy clays - which show increasing degrees o f soil
development (see Table 3.2) in tephric parent materials with the passage of time.
Consequently we are now ready to retrace o ur steps and return to the Manukau
Harbour, and beyond, to the old, weathered c1ay·rich soils of Northland.
Ringp/ain and Steep/ands
81
Distinguishing features of soils of the hill country
The term 'hill country' is widely, but somewhat loosely, used in descriptions of the more rugged
parts of the New Zealand landscape. Soil scientists generally prefer to subdivide hill country into
'hill land' (land with slopes of 12 -28°) and 'steep land' (land with slopes of greater than 28 ~
The soils of the New Zealand hill country are extremely variable. This variability is much greater
than for soils on flat to gently rolling topography. Four interrelated factors contribute to this
variability:
I copogmphic position alld aBe - position on the srope is of more importance than overall
steepness; soil profiles vary greatly depending on whether they are on a:
o ridge crest (soils often shallow because of wind erosion);
D shoulder slope (generally older, more developed soils);
o mid-slope (steeper; soils often eroded and containing rock fragments);
o toe slope (soils deeper and rejuvenated by soil movement downslope; often poorly
drained).
2 geology - parent rock variability is strongly reflected in the soil profile since the soils in
steep land have generally developed directly from the underlying rock rather than from
other soil parent material drifting in (e.g. loess, tephra, alluvium). Certain properties of
parent rocks (e.g. texture, hardness, resistance to weathering) and the presence or absence
~: ~:~:i:;n~oundary between soil and bedrock, are Significant in determining susceptibility
3 climate - diverse microclimates exist in hill country. Ridgetops are exposed to wind; southerly
aspects are colder and usually wetter because they are shady; toe slopes adjacent to valley
floors can be affected by frosts and cold air drainage. Above all, plant-available moisture
varies throughout the slope, depending on subtle differences in soil permeability, soil depth,
water table and lateral flow of moisture in the subsoil.
4. veaetation - the A horizon and soil moisture characteristics are dependent on the type
of vegetation. Tree roots can anchor some soils to a slope; if these are replaced by plants
with shallower, weaker rooting systems there is a higher likelihood of soil loss during periods
of water saturation.
CLASSIFICATION AND MAPPING
Traditionally soils on steep slopes (> 28°) have been classified separately from soils on hill slopes
(12 - 28°) and easier slopes because:
o the steepness limits land-use options;
o the dominance of soil.rejuvenating processes gives rise to weakly developed shallow and
stony soils.
For the latter reason these soils were originally termed 'skeletal soils' but more detailed mapping
in the hill country showed that there was a wide range of soils (including quite deep soils) in
these landscapes.
Detailed mapping of this soil variability in hill land and steep land would be an enormous,
ifnot impossible, task since 70% of New Zealand's area (I.e. 18.5 million hal consists of this type
of topography. Most soil maps of steep land convey the variability by shOwing steep/and soil mappina
units which denote a complex situation involving a whole range of soils, most (but not all) on
steep slopes (>28°). Some may indeed be skeletal soils, some may be deep soils with distinct
profiles; some may be eroded, others stable; some may be developed on accumulation sites, others
on ridge lOpS . There will be major drainage differences depending on the soil depth, physical
properties and slope position . Ideally, the map legend outlines the range of soils likely to be found
within any steepland soil mapping unit.
In hill lands the soils are generally deeper and more developed. Although these soils are more
related to soils on flat or gently rolling slopes they are still quite variable; consequently a hill
soil mappina unit is used to indicate soil complexity within the unit. For example, a mapping
unit such as 'Stratford hill soils' will resemble 'Stratford soils' in many features, but will vary in
both slope and aspect; they will also probably be shallower and display a range of soil moisture
properties because of variations in drainage and microclimate.
LAND USE CONSIDERATIONS
The designation of complex hill soil and steepland soil units on maps is of limited scientific value
but of considerable importance in land use. These hill soil and steepland soil associations can be
rated for pastoral, forestry, and soil and water conservation purposes. In general, pastoral and forestry
uses can be sustained with very careful management but many steepland soils would be recommended
for conservation - protection of biota (plant and wildlife habitats), water and soil conservation,
scenery preservation and outdoor recreation. These should be thought of as very positive land uses,
not merely options to be considered when agriculture and forestry are impractical.
Plate 4 . 12
The Westmere soils have 20 cm of a
very dark brown friable topsoil (silt
loam) over 80-100 cm of yellowishbrown 8 horizon (clay loam). They are
well drained and their relatively old
andesitic parent materials have
weathered to halloysite as well as
allophane. Whereas the Egmont soils
are typical volcanic loams, the
Westmere soils have subsoils which are
sticky when wet; they have a firmer
consistence, blocky structure, and
contain well-developed clay coatings
In their chemical and physical features
they have many similarities with the
volcanic loamy clays, such as the
Hamilton and Naike soils (Plate 3.17) of
theWaikato.